This invention is directed to a code-based monitoring technique for determining audience exposure to a signal of interest reproduced on a television or radio set and, in particular, for enabling detection of a code combined with an audio portion of the signal even when the audio of the set is placed in a mute mode.
Various techniques are known for monitoring transmissions from signal sources such as a television station, a radio station, and cable television channels (referred to collectively hereinafter as "broadcast source"). The signal of interest might be a program being broadcast on the air as the monitoring is taking place, it might be a recorded program being played back on a VCR, or it might even be a commercial. The monitoring is carried out to provide information that, for example, reveals the size of the audience tuned to a given broadcast source at a given time of day, determines the total number of people who have seen a program, provides independent validation that a commercial has been broadcast, and so on. Such information is useful for broadcasters, advertisers, etc. As used hereinafter, the term "program signal" is intended to include all signals, be they, for example, an on-air broadcast or one that has been recorded, a show or a commercial about which such information is being collected.
One approach that has been adopted to before such monitoring is to combine the audio portion of a program signal with a code signal. This is disclosed in U.S. Pat. No. 4,718,106. The combined signal is made available, such as by on-air broadcast, to an intended audience. Certain members of the intended audience are provided with a monitoring apparatus that can detect the code portion of the combined signal. Of course, as explained above, that code portion is part of the audio signal. Consequently, when a television set, for example, is placed in what is conventionally known as a mute mode, the audio portion of the program signal is inhibited from producing sound. Therefore, muting also results in suppression of the code signal. In the absence of such a code signal, it is not possible with the prior art techniques to continue monitoring the program signal by relying on the audio portion of the program signal. Consequently, this mute mode creates an unacceptable gap in the capability of the monitoring apparatus to track the program signal.
Continuing to monitor such information even during muting is useful because the viewer can still be watching the program and/or the commercials even though the audio is muted. Also, when commercial validation is being performed (i.e., to check whether a commercial that has been paid for is actually transmitted by the broadcast source), the commercial might coincidentally be shown during muting. In such a case, the monitoring results would be incorrect. In such a situation, obtaining monitoring information during muting would be of critical importance to determine whether or not a commercial which has been paid for has actually been broadcast. Thus, it is highly desirable to maintain the capability of monitoring the program signal even though a mute mode has been actuated.